1. Field of the Invention
This invention relates to an apparatus for examining documents, in particular documents of value, identification or security documents, having at least one excitation device for exciting luminescence light in or on a document to be examined and at least two detector units for detecting at least part of the luminescence light emitted by the document. The invention relates in addition to a corresponding method.
2. Description of the Related Art
To increase forgery-proofness, identification or security documents or documents of value, such as bank notes, are provided with features or printed with suitable security inks containing luminescent substances. These are substances that can be excited to emit light e.g. by light, electric fields, radiation or sound. During authentication testing, the documents to be checked are usually irradiated with light of a certain spectral region and the luminescence light emitted by the luminescent substances of the document are detected. The intensity and/or spectral characteristic of the emitted luminescence light can then be used to ascertain whether the document is authentic or counterfeit.
The reliability of statements about the authenticity of checked documents is highly dependent here on the accuracy with which the spectral characteristic, i.e. color, of the luminescence light is analyzed. Such analysis can be effected for example by spectrometers, but these require relatively high technical effort and high production costs. A simpler solution is therefore to use individual detector units, such as photodiodes or photomultipliers, with different spectral sensitivity. Depending on the spectral characteristic of the luminescence light, the detector units deliver different detector signals, which can then be used for spectral analysis of the luminescence light.
Apparatuses of this type have the disadvantage, however, that the luminescence light detected by the individual detector units generally does not come from exactly the same partial spatial area of the document due to parallactic errors. This makes it impossible to reliably assess the color properties of the luminescence light emanating from a partial area of the document. This is of disadvantage in particular when partial areas with small extensions are to be examined for their luminescence properties, since in this case even small parallactic errors can lead to especially great inaccuracies in the spectral analysis of the luminescence light.
It is the problem of the invention to state an apparatus and corresponding method allowing higher reliability when examining the luminescence properties of documents, in particular documents of value, identification or security documents, while having a simple structure.
The invention is based on the idea that the detector units are disposed one behind the other with respect to the direction of the luminescence light emitted by the document and hitting the detector units. This causes the luminescence light to successively hit the detector units disposed one behind the other and be detected thereby.
The inventive arrangement of detector units permits all detector units disposed directly one behind the other to detect the luminescence light emitted by a common partial spatial area of the document. Any parallactic errors which would occur with a laterally shifted arrangement of detector units are greatly reduced by the inventive arrangement of detector units one behind the other. Statements about the luminescence properties of the document can then be derived with high reliability from the spectral components of the luminescence light detected by the individual detector units.
In a preferred embodiment of the invention, it is provided that at least a first detector unit is permeable to that partial spectral region of luminescence light which is to be detected with at least a second detector unit disposed behind the first detector unit. A first partial spectral region of luminescence light is then detected by the first detector unit, while a second partial spectral region of luminescence light can pass through the first detector unit and is detected by the second detector unit disposed therebehind. The first detector unit acts here as an optical filter before the second detector unit detector unit therebehind. In certain applications, additional optical filters can therefore usually be dispensed with.
The detector units are preferably photodiodes which are disposed one on the other in layers, forming a so-called sandwich diode. This obtains a very compact arrangement of detector units.
The detector units can fundamentally also be elements capable of detecting light by means of other physical detection principles, e.g. by the avalanche effect.
In a further preferred embodiment of the invention, it is provided that the individual detector units are integrated on a common component, in particular a semiconductor component, that includes at least two photosensitive layers, in particular p-n junctions, one detector unit corresponding to each layer, in particular each p-n junction. The small distance between the detector units obtains an especially great reduction of parallactic errors in this embodiment.
The photodiodes or p-n junctions preferably have different absorption edges, the absorption edge of at least a first photodiode or p-n junction being at smaller wave-lengths than the absorption edge of at least a second photodiode disposed behind the first photodiode or a second p-n junction disposed behind the first p-n junction.
Especially simple and reliable derivation of statements about the spectral properties of the detected luminescence light from the detector signals generated by the individual detector units can be effected on the basis of a division of two detector signals and/or the difference of two logarithmized detector signals.